This invention relates generally to process instruments used in industrial process control systems. More particularly, the present invention relates to a flangeless differential pressure transmitter having an integrated primary element.
Industrial process transmitters are used to monitor process variables of industrial process fluids. For example, pressure transmitters are commonly used in the process control industry for sensing pressures or flow rates of chemicals or petroleum products. A pressure transmitter includes a sensor or transducer that produces an electrical output in response to changes in a process pressure. For example, a capacitive pressure transducer produces a varying electrical signal based on the pressure of a process fluid changing the capacitance of the transducer. The electrical signal of the sensor is processed by transmitter circuitry to produce an electrical output that can be monitored as an indication of the pressure of the process fluid. Pressure transmitters also include electronics and circuitry that allow them to communicate with a process control system for either remotely monitoring the electrical output through a control loop or network, or locally such as with an LCD screen.
A typical capacitive-based pressure transmitter is connected to the process fluid through a simple hydraulic system. The hydraulic system comprises one or more hydraulic passageways that are filled with precise amounts of fill fluid, which communicate the process fluid pressure to the capacitive transducer. At a first end of a hydraulic passageway is a sensor diaphragm that serves as a variable capacitor plate for the pressure sensor. At a second end of the hydraulic passageway is a transmitter isolation diaphragm that separates the fill fluid from the process fluid. In pressure transmitters, the transmitter isolation diaphragms can be positioned such that they are co-planar along a mating face on the base of the transmitter. Thus, the transmitter base allows transmitter isolation diaphragms to be extended to an outer surface of the transmitter suitable for mating with a process fluid source. However, since the transmitter isolation diaphragms are typically flush with the mating face of the transmitter base, a flange adapter union or process flange is typically required to join the process fluid source with the transmitter isolation diaphragms in a sealed arrangement. A process flange is a coupling device that provides a means for mechanically securing the transmitter base with a process fluid source. For example, COPLANAR™ process flanges are commercially available from the assignee of the present invention. A typical process flange comprises through-bores for bolted attachment to through-bores of the transmitter base, and threaded through-bores that align with the transmitter isolation diaphragms for joining with process fluid piping. Thus, the process fluid is brought into contact with the transmitter isolation diaphragms through a bolted connection. As the process fluid pressure fluctuates, the process fluid exerts a corresponding force on the isolation diaphragm at the first end of the hydraulic system, which through the fill fluid adjusts the position of a sensor diaphragm of the capacitive pressure sensor at a second end of the hydraulic system. Furthermore, in order to obtain flow measurements from the transmitter it is also necessary to include a primary flow element, such as a venturi tube, orifice plate, pitot tube or flow nozzle, in the process pipe. Thus, connecting a process fluid source to a pressure transmitter often requires multiple additional components, which provide potential leak points and add costs to installing process control systems. Therefore, there is a need for a simplified and less expensive pressure transmitter with a reduced number of potential leak paths.